1. Field of the Invention
The present invention relates to an aluminum alloy extruded product for beat exchangers and a method of manufacturing the same.
2. Description of Background Art
In automotive aluminum alloy heat exchangers such as evaporators and condensers, an aluminum alloy extruded flat multi-cavity tube including a plurality of hollow sections partitioned by a plurality of partitions has been used as a working fluid passage material.
In recent years, the weight of a beat exchanger provided in an automobile has been reduced in order to reduce the weight of the automobile, taking global environmental problems into consideration. Therefore, a further reduction in the thickness of the aluminum alloy material for heat exchangers has been demanded. In the case of the aluminum alloy flat multi-cavity tube used as the working fluid passage material, since the cross-sectional area is reduced accompanying a reduction in the thickness, the extrusion ratio (cross-sectional area of extrusion container/cross-sectional area of extruded product) is increased to several hundred to several thousand during the manufacture. Therefore, a material having a further improved extrudability has been demanded.
A fluorine-containing compound (fluorocarbon (flon)) has been used as the refrigerant for heat exchangers. However, use of carton dioxide as an alternative refrigerant has been studied in order to deal with global warming. In the case of using carbon dioxide as the refrigerant, since the working pressure is increased in comparison with a conventional fluorocarbon refrigerant, it is necessary to increase the strength of each member of the heat exchanger. Therefore, a material exhibiting high strength after assembling and brazing the heat exchange has been demanded as the working fluid passage material.
Addition of an alloy element such as Si, Fe, Cu, Mn, or Mg is effective to obtain a high-strength aluminum alloy material. However, if Mg is included in the material, when performing inert gas atmosphere brazing using a fluoride-type flux, which is mainly used as the brazing method when assembling an aluminum alloy heat exchanger, Mg in the material reacts with the fluoride-type flux to reduce the degree of activity of the flux, whereby the brazeability is decreased. If Cu is included in the material, since the operating temperature of the carbon dioxide refrigerant cycle is as high as about 150° C., the intergranular corrosion sensitivity is increased.
Therefore, attempts have been made to improve the strength by adding Si, Fe, or Mn to a pure Al material. However, when Mn and Si are added at a high concentration, Mn and Si dissolved in the aluminum matrix increase the deformation resistance, whereby the extrudability is significantly decreased in comparison with a pure Al material when the extrusion ratio reaches several hundred to several thousand as in the case of the extruded flat multi-cavity tube. Extrudability is evaluated by using, as indices, the ram pressure required for extrusion and the maximum extrusion rate (critical extrusion rate) at which the flat multi-cavity tube can be extruded without causing a deficiency at the partition of the hollow section of the flat multi-cavity tube. When Mn and Si are added at a high concentration, the ram pressure is increased in comparison with a pure Al material, whereby the die easily breaks or wears. Moreover, since the critical extrusion rate is decreased, productivity becomes poor.
A method of improving extrudability of an Al—Mn alloy for a photosensitive drum used for a copying machine or the like by reducing the deformation resistance by making the distribution of Mn uniform and causing Mn to coarsely precipitate to reduce the amount of dissolved Mn by performing two stages of homogenization treatment has been proposed (see Japanese Patent Application Laid-open No. 10-72651). However, even if this material is applied as the fluid passage material for automotive heat exchangers, since Mn is caused to coarsely precipitate, the precipitated Mn is redissolved to only a small extent. Therefore, an increase in the strength of the fluid passage material due to redissolution of Mn after assembly and brazing cannot be expected.
In the case of manufacturing a piping aluminum alloy tube for automotive heat exchangers such as automotive air conditioners by a porthole extrusion method using an Al—Mn alloy, Mn-containing compounds precipitate to a larger extent in the end section of the head section of a billet during extrusion of one billet. When continuously forming a joint by attaching the subsequent billet to the preceding billet, the and section of the preceding billet in which the Mn-containing compounds precipitate to a larger extent forms a deposition section at the joint, and the head section of the subsequent billet in which the Mn-containing compounds precipitate to a smaller extent forms a section other than the deposition section. This causes the difference in the precipitation state of the Mn-containing compounds between the deposition section and the section other than the deposition section, whereby the deposition section at a lower potential is preferentially corroded under a corrosive environment. To deal with this problem, a method of preventing the deposition section from being preferentially corroded by causing Mn-containing compounds to coarsely precipitate in the ingot matrix by subjecting an Al—Mn alloy having a specific composition to two stages of homogenization treatment to rescue the difference in the amount of dissolved Mn between the bead section and the end section of the extruded billet to eliminate the difference in the precipitation state of the Mn-containing compounds between the deposition section and the section other than the deposition section has been proposed (see Japanese Patent Application Laid-open No, 11-172388). However, since this method also causes Mn to coarsely precipitate, the precipitated Mn is redissolved to only a small extent. Therefor % an increase in the strength of the fluid passage material due to redissolution of Mn after assembly and brazing cannot be expected.
As a method of manufacturing an aluminum alloy extruded product for automotive heat exchangers, a method of applying an aluminum alloy which contains 0.3 to 1.2% of Mn and 0.1 to 1.1% of Si, has a ratio of Mn content to Si content (Mn %/Si %) of 1.1 to 4.5, and optionally contains 0.1 to 0.6% of Cu, with the balance being Al and unavoidable impurities, and homogenizing the ingot in two stages consisting of heating at 530 to 600° C. for 3 to 15 hours and heating at 450 to 550° C. for 0.1 to 2 hours in order to improve extrudability has been proposed (see Japanese Patent Application Laid-open No. 11-335764). It is confirmed that the extrudability is improved to some extent by using this method. However, since the extrudability is not necessarily sufficient when extruding a thin flat multi-cavity tube as shown in FIG. 1, room for further improvement still remains in order to reliably obtain a high critical extrusion rate.